Nanophotonic Devices for Quantum Optics

1. Feb 13, 2013 GCOE symposium Nanophotonic Devices for Quantum Optics Takao Aoki Waseda University

2. Atom-Light Interaction Interaction between a single two-level atom and single-mode near-resonant monochromatic light: • Strong optical nonlinearity at the single-photon level. • Generation of non-classical light states. • Quantum manipulation of atom/light states.

3. Atom-Light Interaction Interaction between a single two-level atom and single-mode near-resonant monochromatic light: It had been extremely difficult to “isolate” individual atoms and single-mode light from the environment.

4. Interaction of Light and a Single Atom in Free Space Resonant scattering cross section in the weak-driving limit To control only the atom: Just use strong enough light.

5. Interaction of Light and a Single Atom in Free Space Resonant scattering cross section in the weak-driving limit: To control both the atom and light: • Tightly focus the light beam down to . • Confine light in a small volume

6. Interaction of Light and a Single Atom in Free Space Resonant scattering cross section in the weak-driving limit: To control both the atom and light: • Tightly focus the light beam down to . • Confine light in a small volume

7. Tightly Focusing Laser Beam Numerical Aperture (NA) Required NA of the lens ~ 0.65 * NA ~ 0.93 for a clear aperture of 3x beam radius

8. Technical Difficulties Single (laser-cooled) atom in vacuum: hard to trap within a volume ~ l3 Single solid-state emitters (molecule, quantum dot, …): suffer from dephasing due to interaction with phonons In both cases, just detecting a single emitter had been a challenging task.

9. Experimental Progress “Collisional Blockade” Collisional Blockade No Blockade (Poisson Law)

10. Experimental Progress Nature 411, 1024 (2001)

11. Measurement of light-extinction by a single atom Nature Physics 4, 924 (2008) Light extinction (coupling between one atom and a single-mode light beam)

12. Single Photon Source Science 309, 454 (2005) Single-atom Rabi oscillation

13. Single Photon Source Nature 440, 779 (2006) Imperfect interference due to mode mismatching

14. Remaining Problems Collection efficiency into a single-mode fiber < 1% Collection into lens aperture Transmission through various optics Coupling into single-mode fiber ~10% ~50% ~10% High collection efficiency of single photons into a single-mode fiber is demanded.

15. Optical Nanofiber Pull in both direction Microtorch or heater Commercial single-mode fiber r0 = 62.5 mm r(z) rmin < l z Field Intensity F. Warken et al., Opt. Express 15, 11952 (2007)

16. Optical Nanofiber Excitation Collection Efficiency =

17. Atom-NanofiberInterface

18. Achievements at Kyoto T. Aoki, JJAP 49, 118001 (2010) r0 = 62.5 mm r(z) rmin~ 200 nm z single-mode waveguide (silica core, vacuum clad) single-mode fiber (silica core, silica clad) tapered region: multi-mode waveguide Adiabatic condition: (longer taper has lower coupling to higher-order modes, thus shows higher transmission) With tapering length of ~4 cm, we have fabricated tapered fibers with transmission > 99%, which is the highest value ever achieved to date.

19. Our Idea: “Lensed” Nanofiber Nanofiber with a spherical tip = “Lensed” nanofiber

20. Preliminary Study at Kyoto (Numerical Simulations) -10l 10l -5l 5l -2l 2l

21. Preliminary Study at Kyoto (Fabrication) Acknowledgement: I would like to thank Mr. M. Kawaguchi (currently at Dept. of Chem.) for his assistance in the early stage of this work.

22. Interaction of Light and a Single Atom in Free Space Resonant scattering cross section in the weak-driving limit: To control both the atom and light: • Tightly focus the light beam down to . • Confine light in a small volume

23. Interaction of Light and a Single Atom in Free Space Resonant scattering cross section in the weak-driving limit: To control both the atom and light: • Tightly focus the light beam down to . • Confine light in a small volume

24. 2 g 2 G = k Enhancement of Spontaneous Emission • Atom-Light Interaction • Dissipation of Atomg • Dissipation of Lightk Decay rates for g • free space • cavity mode Purcelleffect Enhancement of spontaneous emission if G > g .

25. Silica microtoroidal cavities 10 ～ 100 mm Monolithically fabricated on a Si chip High Q factor（107～1010） High coupling efficiency to optical fibers（～99.9%） D. K. Armani et al.,Nature 421, 925-929 (2003).

26. Cesium atom Placing an atom in the evanescent field S. M. Spillane et al.,PRA 71, 013817 (2005).

27. Realization of strongly-coupled toroidalcQED system Nature 443, 671 (2006)

28. Realization of strongly-coupled toroidal cQED system Nature Physics 7, 159 (2011)

29. One-dimensional system Science 319, 1062 (2008)

30. One-dimensional system PRL 102, 083601 (2009) out in photons out atom “Routing of Single Photons”

31. Achievements at Kyoto T. Aoki, JJAP 49, 118001 (2010) Si substrate SiO2 disk CO2 laser irradiation Photolithography & etching We have achieved cavity Q factor as high as 3x108.

32. Cesium atom Single Atom Trap in the Toroid’s Mode S. M. Spillane et al.,PRA 71, 013817 (2005).

33. Summary • We have proposed novel nanophotonic devices for quantum optics. • Numerical simulations show that a lensed nanofiber has focusing capability and ~30% collection efficiency, and a cleaved nanofiber has ~40% collection efficiency. • We have successfully fabricated lensed nanofibers and cleaved nanofibers. • We have fabricated ultra-high-Q microspherical resonators on a Si chip, which is more suitable for cQED experiments than microtoroidal resonators in terms of mode identification.